The present invention relates to an adjustable valve drive device of an engine, and in particular relates to a technology for properly reducing a stress acting in a direction of separation of first and second split bodies that constitute a rocker cam.
It is generally known that a valve timing or a valve lift of intake and exhaust valves of an engine are changed according to an engine operation state. An example of such an adjustable valve drive device is disclosed in Japanese Patent Laid-Open Publication No. 2004-301058, in which an outer ring is provided so as to fit onto an eccentric cam that is provided at a cam shaft, a rocker cam operative to lift (drive) an intake valve is supported at the cam shaft so as to be rocking, the displacement of the outer ring according to rotation of the eccentric cam is transmittable to the rocker cam via a link mechanism that comprises a control link coupled to the outer ring and a link coupled to the rocker cam, and the distance between a rocker support point of the control link and an axial center of the rocker cam is changeable according to the engine operation state.
According to the above-described adjustable valve drive device, the valve opening start timing can be advanced and the valve lift can be increased by reducing the above-described distance at an engine high-speed and high-load state, while the valve opening start timing can be delayed and the valve lift can be reduced by increasing the above-described distance at an engine low-speed and low-load state.
Herein, the rocker cam for the control of the valve timing of the engine needs to be replaced with a new one when its cam profile face has come to wear off in order to maintain the proper performance.
In a multi-cylinder type of engine, however, since a plurality of rocker cams are provided for multi-cylinders, the replacing of the plural rocker cams may need rather troublesome works. For example, when an rocker cam that is located at the center of the engine is replaced, all adjacent rocker cams to this center rocker cam need to be removed first before the center rocker cam is removed from the cam shaft. Meanwhile, it is preferable in the multi-cylinder engine that the eccentric cam be integrally formed with the cam shaft to ensure a precise distance between adjacent cylinders from an aspect of proper assembling.
From the above-described perspectives, it may be considered that the rocker cam is comprised of a first split body with a cam profile face and a second split body without a cam profile, which are joined to one another by a fastening bolt.
Herein, in general, since a tappet for driving (opening) a valve or a cam follower of a rocker arm contact the cam profile face of the rocker cam, there occurs a relatively large acceleration to the rocker cam at a rising initial timing of the cam profile face, i.e., at a transient period from its base circle area to its cam nose area. Accordingly, in a case where the above-described link mechanism is coupled to either one of the first and second split bodies via a connection pin, an improperly large stress is generated between the first and second split bodies when the engine runs at a high speed. This may cause a concern that a joint split face of these bodies opens or slides.
Increasing of a fastening force of the above-descried fastening bolt or thickness of the split bodies for solving the above-described concern may cause another issue of a large size or a heavy weight of the rocker cam instead. Herein, it is preferable that the weight of the rocker cam be as light as possible because of the rocker cam's repeated rocking movement.